well one of our cat is 9 years old and she never had any problems with her teeth - I feed her wet food and she always has some kibbles. She is an outdoor cat tho and is a real hunter so I guess she keeps her teeth clean by munching on mouse and birds..
Our other cat is 6 yrs and is an indoor only. She eats only kibbles (Orijen and Felidae) and doesn't want to eat anthing else tho i would be more happy if she did - she doesnt even eat meat. She never had any problems with teeth or others...
Our dog is 11 and he almost never gets kibble - he gets home cooked (doesn't eat raw meat) and occasional bones and kibble and my vet said his teeth are as he was 6 yrs
So, I guess kibbles are not that important to keep your animals teeth clean...I also think it has a lot to do with genetics and i also read some study (but don't remember where) about wet food being actually better for your dogs (cats too i suppose) teeth than kibbles...